Capacitative switching apparatus



March 24, 1959 J. M.IHANERT 2,878,768

CAPACITATIVE SWITCHING APPARATUS Filed Oct. 3, 1956 United States Patent2,878,708 CAPACITATIVE SWITCHING APPARATUS John M. Hanert, Des Plaines,Ill., assignor to Hammond Organ Company, Chicago, 111., a corporation ofDelaware Application October 3, 1956, Serial No. 613,700 1 Claim. (Cl.841.08)

The invention relates generally to capacitative switching apparatus andmore particularly to an improved capacity switch mechanism which isthoroughly shielded when the switch is in open position. The inventionis more particularly concerned with an improvement in capacitativeswitching means for electrical musical instruments in which the outputof a signal generator is transmitted to the output system of theinstrument through such capacitative switch means.

It is an object of the invention to provide a capacitative switchingmeans in which the switching parts are thoroughly shielded when theswitch is open and in which depression of a playing key causes acapacitative coupling between the source of tone signals and the outputsystem of the instrument.

A further object is to provide a capacitative switching means forelectrical musical instruments in which the signals from generator ofthe unison pitch together with those from the generators of octavelyrelated pitch are successively transmitted to the output system of theinstrument as the playing key is being depressed.

Other objects will appear with the following description, referencebeing had to the accompanying drawing in which:

Fig. 1 is a partially diagrammatic and partially schematic wiringdiagram of a typical portion of an electrical musical instrument;

Fig. 2 is a broken vertical on the line 22 of Fig. 1;

Fig. 3 is an enlarged plan view of the movable element of the capacityswitch showing schematically its connection to the tone signalgenerators;

Fig. 4 is a sectional view of the capacitative switch element taken onthe line 4-4 of Fig. 3; and

Fig. 5 is a bottom plan view of the capacity switch plate element shownin Figs. 3 and 4.

In electrical musical instruments it has been endeavored frequently tocouple the output of the signal generator to the output system of theinstrument through a key operated capacity switch. Such proposals havenot met with acceptance primarily because of the existence of a certainamount of coupling between the signal source and the output system dueto coupling between the respective plates of the capacitor switch whenthe switch was in open position. In the present invention suchundesirable residual coupling is avoided by providing a substantiallycomplete ground-connected electrostatic shielding for the switch when itis in open position, thus preventing any substantial coupling betweenthe capacitor plates of the switch when the switch is in unoperatedposition.

In Fig. l the apparatus is diagrammatically illustrated as including aplaying key representative of the sixtyone, more or less, keys of anelectrical musical instrument such as an organ. Each of the playing keys10 is secured to a key shank or channel 12 which at its rearward end issecured to the upper end of a flexible leaf spring 14. The spring 14 isfixed at its lower end to a stationary part 16 of the keyboard frame andat its upper end to a sectional view thereof taken 2,878,708 PatentedMar. 24, 1959 deformable angle clip 18, which in turn is secured to theshank or channel 12. The spring 14 is stressed so as normally tomaintain the forward end of the key 10 in engagement with a stationaryfelt upstop 20. By bending the angle clips 18 the effectiveness of thesprings 14 may be readily adjusted to provide the desirable touch orresilient resistance to depression of thekey 10. Each of the key stemsor channels 12 has secured to it at its rearward end a verticallyextending arm 22. At its upper end the arm 22 has secured thereto acondenser plate 24, which upon operation of the key 10, is adapted toproject through a slot, or opening, 26 formed in a shielding housing 28and into a slot 30 formed in a condenser plate 32 which is supported inthe housing 28 by insulating means 34. The slotted plate 32 extends thefull length of the keyboard. The shielding housing 28 is supported byany suitable part 36 of the keyboard frame and has its capacitor plateconnected to a conductor 38, the housing 28 and a shield 40 for theconductor 38 are respectively connected to ground.

The conductor 38 is connected to ground through a capacitor C42 and gridresistor R44 which provide sink and grid return circuits for the inputsignals which are impressed upon the grid 46 of a preamplifier triode48. The cathode of triode 48 is connected to ground through a suitableself-bias mesh comprising capacitors C50 and resistor R52, while theanode or plate thereof is connected through a load resistor R54 to asuitable source of plate current supply indicated as a terminal B+ ofthe customary power supply. The plate is also connected through ablocking capacitor CS6 to the input of a power amplifier 58, the outputof which is connected to a speaker 60 The mounting for the keys 10 issuch that after the keys have been depressed to a certain limited extenta spring 66 secured beneath each of the keys comes into contact with astop pad 68, suitably mounted on a fixed frame member 70, so thatfurther depression of the key will require the application of additionalforce so as to overcome not only the force of the spring 14 but alsothat of the spring 66. The relative strength of the springs 14 and 66should be such that when the key has been sufficiently depressed tocause the spring 66 to become effective, the player will readily sensethe ditference in force required to depress the key against the force ofboth springs, and thus readily play the instrument using the techniqueof second touch.

The body of the capacitor plate 24 is made of a suitable dielectricmaterial and has applied thereto suitable separated areas, or islands,of conducting material as shown in Figs. 3, 4, are electricallyconnected to ground being joined along the edges of the plate 24. Whenthe key 10 is in normal position, as shown in Fig. l, the conductiveislands thus form, together with the housing 28, a substantial completeelectrostatic shield, preventing capacitative coupling between otherconductive areas of the plate 28 and the slotted plate 32. Conductiveareas 74 and 74a, connected together along one edge of the dielectricplate 24, are suitably connected to a generator 1 preferably providingcomplex musical tone signals of the pitch of the unison or fundamentalfrequency of the tone to be controlled by each particular key.Similarly, conductive areas 76 and 76a, which are likewise electricallyconnected, are connected to a tone signal source, or generator, 2 whichprovides the octave tone signals. Also, if desired, conductive areas 78and 78a may be similarly connected to the generator 4 of tone signalstwo octaves higher in pitch than the unison.

The slot 30 in the condenser plate 32 is sufficiently wide to provideadequate clearance for the entry of the capacitor plate 24 and to permitthe slight relative tilting 3 action of the plate 24 with respect to thehorizontal walls of the slot 30. The degree of capacitative couplingbetween the conductive surfaces on the plate 24 and the respectivesurfaces of the slot 30 of the condenser plate 32 may vary somewhat dueto the pivotal mounting of the plate carrying arm 2t) and due tounavoidable inaccuracies in construction, but the total coupling willnot be affected by such variations. The degree of capacitative couplingwill be afiected only by the extent to which the capacitor plate 24isinserted in the slot 30.

As previously indicated, the capacity switch is completely shielded bythe grounded housing 28, and the capacitor plate 24 has groundedconducting portions which form, with the edges of the slot 26 in thishousing, a substantially complete electrostatic shielding means, toprevent any capacitative coupling between the signal carrying theconductive areas on the dielectric platea24 and the slotted plate 32.However, when a key is partially depressed the conducting portions 74 onthe plate 24 gradually are moved to a position to provide electrostaticcoupling between the signals provided by the source 1 and the plate 32so as to impress signals of unison pitch upon the control grid of thetriode 38 which, being amplified, is translated into sound by thespeaker 60.

When substantially all of the conducting area 74, 74a lie within theslot 36 of the plate 32 the spring 66 of the depressed key will havemoved downward at least a sufficient distance for its lower end toengage the stop 68, and the force required for further depression of thekey will thus be increased. Thereafter, further depression of the keyagainst additional resilient force of the spring 68 will cause theconducting portions '75, 76a of the plate 24 to be moved intocapacitative coupling relation with the plate 32, and upon completedepression of the key the conducting area 78, '78:: will be brought intocapacitative relation with plate 32. Thus, signals from the generator 2and the generator 4 will be combined successively with the output of thegenerator f.

It will be noted that the conducting areas 72, 74, 76, and 78, and theirobverse parts are spaced so as to be adequately insulated from oneanother. These conducting areas may be formed on the dielectric plate 24in the form of sheet metal foil applied to the plate with suitableadhesive, or may be produced by any of the well-known printed circuittechniques.

This capacitative switch means differs from others known to applicant inthat the movable part of the switch has a grounded portion normallyproviding a shielding bridge across the opening through which themovable part projects so that, together with the housing of the switch,the parts thereof are thoroughly shielded. Another feature of theinvention is that upon operation of the key to the extent necessary toprovide capacitative coupling of the source of the signal of unisonpitch, the resilient means becomes effective to require noticeablyincreased force to effect continuation of key depression, and at thesame time, the switch becomes effective to couple capacitatively theoutput of additional tone signal sources which are harmonically relatedin pitch to that of the generator first coupled to the output system bythe capacitative switch. Of course, if desired, additional harmonicallyrelated sources of electrical tone signals might be coupled to theoutput system by adding conductive areas such as 76 and 78 to themovable plate of the capacity switch.

It will be clear to those skilled in the art that the shape of themovable plate 24 is not necessarily rectangular in cross section butthat it should be of such conformation as to provide a shield for theopening in the housing 28 by virtue of its grounded end conductingportion. While the slotted shape of the fixed plate 32 is of advantagein providing compensations for inaccuracies in construction and for thearcuate movement of the plate 24, this conformation of the plate 32 isnot essential to the operation of the switch. The more fundamentalfeatures of the invention are that the movable plate 24 has at itsforward end a grounded means for shielding the opening in the housing 28or, more particularly, for providing a shield between the fixedcondenser plate 32 and those conducting portions on the movable plate 24which are connected to signal sources.

While I have shown and described a particular embodiment of myinvention, it will be apparent to those skilled in the art that numerousmodifications and variations may be made in the form and constructionthereof, without departing from the more fundamental principles of theinvention. I therefore desire, by the following claim, to include withinthe scope of my invention all such similar and modified forms of theapparatus disclosed, by which substantially the results of the inventionmay be obtained by substantially the same or equivalent means.

I claim:

For use in an electrical musical instrument having a plurality ofmusical tone signal sources, each source having one of its outputterminals grounded, a pivotally mounted playing key, and an amplifyingoutput system having one of its input terminals grounded, theimprovement which comprises a capacitative switching apparatus coupledbetween the ungrounded output terminal of each signal source and theungrounded input terminal of the amplifying output system and comprisingin combination, a grounded shielding housing having an opening therein,a capacitor plate mounted within the housing adjacent the openingtherein and insulated from the housing and coupled to the ungroundedinput terminal of the amplifying output system, a dielectric elementmechanically connected to the key and movable a substantial distanceinto the housing through the opening therein upon full depression of thekey, the portion of the element which penetrates the housing havingmeans forming a plurality of separate conductive surfaces arrangedsuccessively to come into capacitative coupling relationship with thecapacitator plate as the element is moved into the housing, andmeansconnecting the first of the conductive surfaces which successivelymove toward the capacitator plate to ground and the rest, respectively,to the ungrounded output terminals of the signal sources.

References Cited in the file of this patent UNITED STATES PATENTS 2,137,435 Yolles Nov. 22, 1938 2,215,809 Miessner Sept. 24, 19402,329,544 Larsen Sept. 14, 1943

